1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly, to an image reading apparatus which compensates inter-deviations of an RGB balance which is created when a linear image sensor which comprises three linear image sensors for R, G and B successively reads scanning lines on a color original.
2. Description of the Background Art
As well known, in an image reading apparatus, such as a color process scanner for manufacturing a plate, which digitally reads an image, three linear image sensors are disposed parallel to each other in correspondence to optical color components of red, green and blue (hereinafter "R," "G" and "B") which are to be read. Each linear image sensor is formed by arranging a plurality of CCD elements in a line, and a color filter (R-, G- or B-filter) is disposed on each linear image sensor. The respective linear image sensors read R-, G- and B-components of the original pixel by pixel. Based on image data or a corrected version of the image data regarding each color component which is read in this manner, a recorded image (color-separated image or reproduced color image) is generated.
There is a relative difference between the sensitivities of the R-linear image sensor, the G-linear image sensor and the B-linear image sensor. FIG. 6 is a graph showing such a difference in the sensitivities. In FIG. 6, an incident wavelength is measured along a horizontal axis and a relative sensitivity (which is an output level which is obtained when light having a uniform distribution all over the visible light band range impinges upon the sensors) is measured along a vertical axis. As shown in FIG. 6, the R-linear image sensor is sensitive to light belonging to a wavelength band range of 600-700 nm, exhibiting the highest output level. The G-linear image sensor is sensitive to light belonging to a wavelength band range of 500-600 nm, exhibiting an intermediate output level. The B-linear image sensor is sensitive to light belonging to a wavelength band range of 400-500 nm, exhibiting the lowest output level. That is, the sensitivity of the R-linear image sensor is highest, and the G-linear image sensor and the B-linear image sensor have successively decreasing sensitivities in this order.
Due to such a difference in the sensitivities between the linear image sensors as described above, even when light from a light source which has no inter-deviations of the RGB balance (which is an RGB ratio) is read (that is, even when sunlight is read), the RGB balance is shifted at an output side of the sensors. Further, the inter-deviations in the RGB balance changes as the color filters which are disposed on the CCD elements deteriorate with time. In addition, there is virtually no case where an image is read under natural light, in reality. Rather, in most cases, image reading is performed using an artificial light source (e.g., a halogen lamp or a fluorescent lamp). Further, since artificial light sources have different color temperatures, it is necessary to adjust the RGB balance in accordance with the color temperatures of the artificial light sources which are used. Still further, since the color temperatures of the artificial light sources change with time, it is preferable to adjust the RGB balance once again every time image reading is performed.
However, no image reading apparatus has yet been realized which compensates such inter-deviations of an RGB balance between the respective linear image sensors as described above while compensating inter-deviations (including change with time) of color temperatures of light sources which are used.
There are some conventional techniques which control a charge accumulating time of a CCD by detecting a change in the quantity of light from a light source. A first group of such conventional techniques is related to an apparatus for reading a monochrome image, and unlike the present invention, is not related to an apparatus which compensates inter-deviations of and a change in color balances of a light source and each linear image sensor. In short, the respective conventional techniques detect only the brightness (luminance) of a light source and control a charge accumulating time of a CCD based on a detected change in the brightness. A second group of the conventional techniques uses means (specifically, a photodiode) for detecting a change in the quantity of light, in addition to CCDs, which makes the structure complex.